The present invention relates generally to MR imaging and, more particularly, to a method and apparatus to generate a substantially circular polarized RF field about a subject independent of subject asymmetry.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The use of RF coils to generate an RF field about the bore of a magnet for imaging is known in the art of nuclear magnetic resonance imaging. Generally, a patient or other imaging subject is positioned on an examination table and inserted into a coil arrangement having a cylindrical bore therethrough. The RF coils extend around the bore and when energized, transmits and/or receives RF energy. In addition to whole body coils, it is known to utilize anatomically directed coils for imaging targeted anatomical regions of a patient. For instance, head coils have been developed and are specifically designed to image the head of a patient.
Head coils, as well as other coils in which the coil elements are arranged in a birdcage arrangement, are generally cylindrical and are designed to generate a substantially circular polarized RF field inside the volume of the coil. With a symmetrical polarized field, the center of the coil, perpendicular to the axis of the coil elements, is typically considered as a virtual electrical ground plane. However, when a patient is placed in the volume of the coil, the asymmetry of the patient will shift the ground plane of the coil and therefore the center of the coil may no longer be relied upon as a good grounding location. That is, the human body is asymmetric and generally distorts the symmetry of the coil especially when only a portion of the patient is positioned within the coil.
Further, in the context of head coils, the shoulders of a patient are placed in contact with the head coil assembly. Such contact may also affect the symmetry created within the volume of the head coil. It is also recognized that at higher frequency imaging, the asymmetries inherent in the human body impact the symmetry of the polarized field within the coil and will worsen since with higher frequency imaging coupling to the patient increases. Moreover, the otherwise substantially circular symmetry of the RF field created within the volume of the coil may also become distorted, and drive ports connected to the coil that are driven at voltages out of phase to one another by 90 degrees, are no longer shifted properly when the coil is loaded with a patient. Hence, quadrature isolation is destroyed as is efficiency resulting in a decrease in SNR as well as an increase in SAR.
A volume coil having sixteen coil elements with a center ground plane is illustrated in FIG. 1. As schematically shown, coil 2 includes an array of coil elements 3 that are uniformly spaced from one another and designed to create a substantially circular polarized field when oriented in a cylindrical arrangement. As mentioned above, with a conventional birdcage coil 2, a center of the coil 4 is considered as the virtual ground. The coils are driven through the application of voltages at two tangentially 90 degree apart drive ports 5, 6. Extending from each drive port 5, 6 is a drive cable 7,8, respectively, The drive cable 7 connected to drive port 5 extends to the superior end-ring 9 of the coil 2, and drive cable 8 extends from drive port 6 to the inferior end-ring 10. The drive voltage applied at drive port 6 is 90 degrees shifted in phase from the voltage applied at drive port 5 so as to set up, absent patient induced asymmetry, the generally circular polarized field inside the volume of the coil. Each drive cable 7, 8 is physically soldered to the substrate of the coil 2 with their shields contacting the center of the coil. The center of the coil 2 is considered the virtual ground, thus killing all standing waves on the cable shield. Notwithstanding the benefits of such a coil design, as noted above, the patient asymmetry may impact the symmetry of the circularized polarized field otherwise created within coil 2. As a result, the field may become more linear than circular thereby introducing shading to reconstructed images. Moreover, with decreased circularity in the polarized field, the power requirements of the coil also increase. Additionally, the linearity resulting in the volume of the coil creates localized high energy fields within the coil volume thereby increasing temperature differentiation across the coil volume.
It would therefore be desirable to have a system and method capable of generating a substantially circular polarized RF field independent of subject asymmetry or incidental subject contact with an RF coil assembly during data acquisition.